Tryptophan Fluorescence as a Structural Probe of Interaction of Amphiphilic Peptides with Calmodulin

Abstract

Several peptides exhibit calcium-dependent high affinity binding (dissociation constant ≃10^-9M) to calmodulin (CaM), the ubiquitous calcium binding protein (Malencik and Anderson, 1983; Malencik & Anderson, 1984; Cox et al., 1985). These peptides, such as the three mastoparans, competitively displace enzymes (e. g. myosin light chain kinase) off the 1:1 CaM-enzyme complex to form the CaM-peptide complex (Malencik and Anderson, 1983). Consequently they are considered to be viable structural models for the CaM-interacting surfaces on the CaM-dependent enzymes. It was proposed independently by us and by others, that the ability to form a basic amphiphilic helix is probably the common driving force for these peptides to bind to CaM (Cox et al., 1985; McDowell et al., 1985). The purpose of this study was to probe the environment of the CaM-interacting region of some amphiphilic peptides by using the fluorescence signal from their single tryptophan (Trp) residues (CaM does not contain Trp). The single letter amino acid sequences of these peptides are shown in Table 1. With the exception of melittin the Trp in each peptide forms a part of the hydro-phobic face of the putative helical wheel (not shown, see McDowell et al., 1985). They all have large helical hydrophobic moments (Eisenberg et al., 1982) and difference circular dichroic (CD) spectra suggest that a high degree of helicity is induced in the peptides upon CaM binding (Maulet and Cox, 1983; McDowell et al., 1985).